Closed mixer working process with stroke-control ram

ABSTRACT

The invention relates to the processing, in a closed mixer ( 1 ), of polymer-based mixtures and compounds for the manufacture of tyres or components thereof.  
     To improve the dispersion of the ingredients in the polymer base, the pressing ram ( 8 ) is lowered to the working condition controlling the position thereof over time, in order to compress the ingredients within the mixing chamber ( 4 ).  
     The invention is particularly suitable for the dispersion of reinforcing fillers such as silica or carbon black in the polymer base and the incorporation of said fillers in the latter.

[0001] The present invention generally relates to the processing ofpolymer compounds and mixtures in a mixer of the closed type.

[0002] This kind of processing is commonly performed in the manufactureof tyres or components thereof such as tread bands and the like; suchprocessing consists of mixing a polymer base together with otheringredients according to predetermined recipes, in order to obtainsemi-finished products to be used in the subsequent phases of themanufacturing cycle.

[0003] Closed mixers, also known as discontinuous mixers, aresubstantially formed by a mixing chamber accommodating a pair of rotorsrotating in mutually opposite directions, wherein the material is loadedin predefined quantities (batches) and processed by the rotors whichcompletely discharge it from the mixer at the end of the cycle, beforebeginning to process a new batch of material.

[0004] In this connection it should be considered that the term“mixture” in this description is intended to indicated the genericintermediary product obtained during any phase of the processing cycle,while the term “compound” will be used to refer to the semi-finishedproduct derived from the addition of the cross-linking system to themixture in order to prepare it for subsequent vulcanization.

[0005] The chemical-physical characteristics of the mixtures and thecompounds, which allow an end product with desired qualities to beobtained, depend also on the processing in the closed mixers; for thisreason the control of this operating phase has a decisive role in themanufacture of tyres and their components.

[0006] The process variables which influence the working process inclosed mixers are many and it is therefore difficult to ensure anadequate control thereof throughout the whole operating cycle, i.e. fromthe filling of the mixer to its emptying.

[0007] Examples of these process variables are the characteristics ofthe ingredients used for compounds and mixtures, their temperature, thespeed and the torque applied to the rotors, the filling level of themixer (i.e. the so-called “fill factor”) and others.

[0008] Nowadays various mixing control systems are known, which arebased on the detection over time of certain process parameters (alsocalled “indirect” parameters), such as the temperature of the mixturesor the energy conferred to the system, and on the eventual correctionthereof by modifying other parameters (also called “direct” parameters)which directly determine the functioning of the mixer, such as, forexample, the speed of the rotors, the pressure of the pressing ram andso on.

[0009] Examples of this procedure are described in international patentapplication No. PCT/US98/23294 and in U.S. Pat. No. 4,830,506.

[0010] In particular the latter describes a system for controllingprocessing in a closed mixer, based on regulating the pressure of thepressing ram so as to ensure that when the ram is in the loweredoperating position, it can oscillate with a small predeterminedamplitude (in the order of half an inch, i.e. approximately 12.5 mm).

[0011] According to this U.S. patent, with such a control system it ispossible to obtain an improved mixing action by making use of theperiodic rebounds due to the rotation of the rotors.

[0012] One characteristic of compounds and mixtures which, untilnowadays, closed mixers and their regulating systems did not controlefficiently, is the dispersion of the various ingredients within theprocessed mass.

[0013] This characteristic is of considerable importance for obtainingan end product with the required qualities, both because anon-homogenous dispersion of an ingredient in the mixed material cancause local defects in the tyre or semi-finished product which will bemanufactured, and because a different dispersion between one charge ofmaterial processed in the mixer and the subsequent one, can result inlack of uniformity between “batches” with the same basic formulation andcan negatively influence the repeatability of the rheometric curve ofthe compound.

[0014] The present invention aims at providing a method for regulatingthe working process in a closed mixer, which allows a better and moreconstant dispersion of the ingredients in the processed mass to beobtained.

[0015] The invention arises from the Applicant's perception that saiddispersion also depends on the profile of the position over time of thepressing ram, during the phase of incorporation of the materials in thepolymer matrix.

[0016] The invention applies preferably, but not exclusively, to theincorporation phase of the reinforcing filler in the polymer; as“reinforcing filler” is intended here both the carbon black commonlyused in the manufacture of tyres and the so-called white fillers withtheir optional bonding agents.

[0017] Said fillers are ingredients of the inorganic type such asgypsum, talc, kaolin, bentonite, titanium dioxide, alumina, varioussilicates and silica, which are used for tyre compounds in order toincrease grip in the wet conditions, reduce the rolling resistance ofthe tyre and other purposes.

[0018] The invention also applies to the addition of the crosslinkingsystem to the mixtures in order to obtain compounds to be vulcanized, aswell as to the mixing of mixtures having different compositions and,more generally, whenever it is required to combine the polymer matrixwith substantially solid material introduced into the mixer duringprocessing.

[0019] According to the invention, the downward stroke of the ram afterthe introduction of the material into the mixer is performed bycontrolling the descent position-time profile thereof during theincorporation phase in the polymer, in such a way as to follow apredetermined reference pattern: this allows to achieve a controlledposition of the ram, which gives time to the rotors for mixing theingredients before the ram reaches the working condition at the end ofits stroke.

[0020] The term “position-time profile” in this description and in thefollowing claims, is intended to refer the profile over time of theposition of the pressing ram; said profile can easily be representedgraphically, by plotting in a Cartesian plane the position of the ram onthe ordinate and the time on the abscissa.

[0021] Further features and advantages of the invention will emerge moreclearly from the description given here below with reference to theaccompanying drawings, of a preferred but non-exclusive embodimentthereof.

[0022] In particular, the first figure of the drawings shows a mixer ofthe closed type for carrying out the method of the invention.

[0023]FIG. 2 is a graph showing the profile over time of the position ofthe pressing ram, during a working cycle according to the invention.

[0024]FIG. 3 is a graph showing the profile over time of the powerapplied to the mixer and the operating pressure of the pressing ram,during the same working cycle of FIG. 2.

[0025]FIG. 4 is a table showing the position of the pressing ram as afunction of time, during the phase of incorporation of the filler in thepolymer matrix which takes place in the processing cycle of FIGS. 2 and3.

[0026]FIG. 5 shows an explicative graph of the effects, on dispersion,of the method according to the invention.

[0027] Last, FIGS. 6 and 7 are the rheometric curves of compoundsprocessed, respectively, with the method of the invention and accordingto the prior art.

[0028] With reference to the first figure, the mixer 1 shown therein isof the so-called “Banbury®” type which comprises a pair of tangentialrotors 2 and 3 rotating in opposite directions inside a mixing chamber4, cooled by water circulating in a series of channels 5, 6.

[0029] The mixer 1 comprises the pressing ram 8 fixed to the end of arod 9 actuated by a pneumatic (or hydraulic) cylinder 10.

[0030] The mixer 1 is further provided, for introduction inside it ofthe material to be processed, with a lateral hopper 11, whereasdischarging of the processed mixtures or compounds takes place frombelow, through an opening present in the base of the mixing chamber 4and closed by an obturator device 12.

[0031] In the mixer 1 the position of the pressing ram 8 and the rod 9which actuates it is detected in a way known per se with transducers,not shown in the drawings; according to the invention said position isdetected throughout the whole stroke of the pressing ram, from theraised (resting) condition thereof in which it allows charging of thematerial via the hopper 11, to the lowered (working) condition.

[0032] As lowered (working) condition is intended the distance betweenthe point at which the pressing ram meets the compound and the lowestpoint that can be reached by said ram, depending on the filling level ofthe mixing chamber and on the physical characteristics of the compoundbeing processed. These two extremes are respectively called the upperand lower end-of-stroke positions.

[0033] In this way it is possible to determine the downward speed of thepressing ram throughout its stroke and to regulate said speed, as willbe explained more clearly hereinafter, by means of the general controlsystem of the mixer which for this purpose comprises electronic means(of the PLC type and the like) known per se.

[0034] The graphs in FIGS. 2 and 3 refer to the processing of a typicalmixture for compounds reinforced with silica, whose the composition isgiven below by way of example, specifying the ranges of variation foreach ingredient and expressing the quantities in “phr”, i.e. parts byweight for each 100 parts of polymeric material: Polymer base 100 Carbonblack  0-80 Silica 10-80 Bonding agent (of the silica) 4%-15% of thesilica Zinc oxide (ZnO) 1-3 Stearic acid 0-3 Anti-deteriorating agents1-3 Plasticizing oil  0-30 Anti-ozone wax 0.5-3   Specific chemicalingredients  0-15

[0035] In order to obtain the compounds, it will be necessary to add tothese ingredients the crosslinking system in the usual quantitiesaccording to the composition of the mixture; said system is preferablycomposed of sulphur (from 0.5 to 2.5 phr) and vulcanizationaccelerators.

[0036] The polymer base can be any polymer or mixture of polymers,either of natural or synthetic type, suitable to assume all thechemicophysical and mechanical characteristics of the elastomers afterappropriate crosslinking.

[0037] Preferred polymer bases include polymers or co-polymers with anunsaturated chain, obtained through the polymerisation of conjugateddienes and/or aliphatic or aromatic vinyl monomers.

[0038] In the example referred to in FIGS. 2 and 3, the polymer massweighs 113.7 kg (with a dry content equal to 94.08 kg) and is mixedtogether with a reinforcing filler comprising, in addition to carbonblack, silica.

[0039] The overall weight of the reinforcing filler is 68.2 kg, whereinthe silica constitutes 58% which is bonded to the polymer base by meansof known elements such as silane.

[0040] In the example, the processed mass also comprises 8 kg of freeoil and 21.15 kg of other ingredients present in smaller percentagessuch as, typically, silane or other bonding agent of silica, zinc oxide,stearic acid and anti-deteriorating agents, in such a way as to obtain afill factor of the mixer equal to 67.7%.

[0041] The latter is defined by the ratio between the mass present inthe mixer (211 kg in the example) and the capacity of the mixer (equalto 270 liters) multiplied by the density of the aforementioned mass(1.154 kg/dm³).

[0042] The processing cycle referred to in the graphs of FIG. 2 and 3,is divided into successive phases which can be identified from the timeon abscissa.

[0043] In this way at the start, the rotors 2 and 3 of the mixer 1 arebrought up to a predetermined average speed of 40 revolutions/minute,for the initial introduction of the ingredients into the hopper 11(interval from 0 to 2 min. about); in this condition the pressing ram 8is fully raised to allow the passage of the ingredients towards themixing chamber 4.

[0044] Once the loading phase has been completed, the ram 8 is lowered:this operation is performed by controlling the pressure applied by theactuating fluid (oil or gas) on the pressing ram, in such a way as tofollow a predetermined absolute position-time.

[0045] The subsequent positions of the ram as a function of time(calculated in seconds from the start of the descent of the ram) aresummarised in the table in FIG. 4, and the data relating thereto isshown in graphical form in FIG. 2.

[0046] As can be seen, in this example initially the descent occurs veryquickly (over 1,250 mm in 6 seconds), until the ram meets the mass ofthe ingredients introduced into the mixer.

[0047] At this point, control of the pressure applied by the operatingfluid on the pressing ram is initiated, as a function of the positionthereof: the ram begins therefore to descend in gradual manner with acontrolled feed movement of 90 mm in 45 seconds.

[0048] In this connection it is worth to explain that reference is madeto an average forward movement of the ram, because the real movementthereof is characterised by successive oscillations (visible in thediagram of FIG. 2) caused by the mass introduced into the mixingchamber, which is subject to a periodic movement imparted by therotation of the rotors that superposes to the downward linear movementof the ram.

[0049] The position of the latter is regulated by adjusting the pressureof the fluid in the actuating cylinder 10; this regulation is controlledby the mixer control system, which in this phase regulates the pressureapplied to the pressing ram and causes said ram to move downwards with apredetermined position profile, whose gradient (i.e. the degree ofdisplacement per unit of time) is set out in the recipe.

[0050] In this way the position over time of the ram is used as aprocess control parameter, in addition to those parameters alreadyknown, such as temperature, pressure applied to the pressing ram, power,etc.

[0051] The abovementioned position also supplies information on thecompaction curve of the material which, as is known, is correlated toincorporation of the polymer in the chamber.

[0052] In this manner it also becomes possible to identify the optimumtime for addition of the plasticizing ingredients, followingincorporation of the filler.

[0053] When the pressing ram 8 reaches the working condition, mixing ofthe mixture proceeds according to the usual criteria which will dependin each case on the semi-finished product to be obtained.

[0054] The graph in FIG. 2 shows the position-time profile of theposition of the ram for an operating cycle; said position is expressedas a distance in millimeters (mm) from a predetermined reference point,which in this case is the lowered working condition, while the time isin minutes.

[0055]FIG. 3, on the other hand, shows the graphs of the pressureapplied to the pressing ram for actuation thereof and the instantaneouspower used by the rotors, during the same operating cycle of FIG. 2; theunits of measurement are psi for pressure, kW for power and minutes fortime.

[0056] As can be seen, in the phases subsequent to that of introductionof the ingredients considered above, the ram is raised and loweredquickly between the upper and lower end-of-stroke positions, because thefurther introduction of other material to be mixed is not sheduled;however, this possibility should not be excluded and in such a case,lowering of the ram will still be performed in controlled mannerlikewise already explained.

[0057] The results obtained with the processing method according to theinvention have been rather favourable.

[0058] Indeed, upon performing laboratory tests on mixture samplesaccording to the above example (using the reflected light technique andthe “Dispergrader 1000” system made by Optigrade), it was possible toascertain that the dimensions and the number of aggregates present inthe processed mass were lower with respect to a mixture obtained withthe same process in which, however, lowering of the pressing ram did nottake place in a controlled manner according to the teaching of theinvention.

[0059] For this purpose, reference is made to the graph in FIG. 5obtained on the basis of the aforementioned tests, where the ordinatesshow the number of aggregates having a specific average diameter, shownon the abscissas.

[0060] As can be seen, the curve relating to the mixture processedaccording to the invention has for each average diameter value a numberof aggregates which is systematically lower than that of the mixtureprocessed normally, i.e. without control of the descent of the pressingram.

[0061] Furthermore, with the method according to the inventionaggregates with an average diameter greater than 50 μm are almostcompletely eliminated.

[0062] Of course variations of the invention with respect to what hasbeen described hitherto, are possible.

[0063] In the first place it is necessary to point out that theprocessing method considered can be used in all mixers of the closedtype (for example also those with intermeshing rotors) provided with apressing ram.

[0064] Furthermore, as mentioned above, control of the downward strokeof the pressing ram must not be regarded as applicable only todispersion of the reinforcing filler, but can also be performed duringall the processing phases which require dispersion of other materials inthe polymer mass, such as the addition of the crosslinking system to themixtures in order to obtain compounds to be vulcanized, or the mixing ofmixtures of different compositions carried out by adding one of them tothe mixture already present in the mixer, and so on.

[0065] However, implementation of the method according to the inventionin relation to the reinforcing filler is particularly advantageousbecause, in addition to the improved dispersion already mentioned above,it also allows addition of the plasticizing substances to be optimised.

[0066] In this respect it is necessary to point out that in the priorart, since the phase of incorporation of the filler is performed withoutcontrolling the position of the ram and with the use of conventionalparameters such as temperature, energy and power, execution thereofoccurs over a predetermined time span (typically in the region of 1min.), at the end of which it is assumed that incorporation iscompleted.

[0067] The result is that, without any control, the ram can reach theworking condition either before or after the abovementioned time span.

[0068] In the first case a loss of specific power occurs: this meansthat there is a dead time which could be used to add the plasticizingsubstances, but which is not used for this purpose because there is notsufficient certainty that incorporation of the filler has finished.

[0069] By lowering the ram controlling its position over time on thebasis of a predetermined feed speed as in the present invention, it ispossible to ensure that the incorporation of the filler finishes exactlywhen the ram reaches the lowered working condition, so as to bring itback into the raised resting condition afterwards and introduce theplasticizers.

[0070] In the second case, on the other hand, since the ram reaches theworking condition too late, a loss of material can occur as a result ofthe suction when said ram is raised to start the subsequent processingphase, for example cleaning thereof or the introduction of plasticizers.

[0071] In both cases mixing of the processed material is affected.

[0072] The consequences of this situation are shown in FIG. 7 comparedwith FIG. 6. They show the rheometric curves relating to compoundsaccording to the above recipe being obtained by controlling the pressingram with the method of the present invention (FIG. 6) and operating inaccordance with the prior art (FIG. 7).

[0073] As can be seen, while in the case of the method of this inventionthe various curves plotted on the graph in FIG. 6 have a rather uniformtrend, in the case of the prior art (FIG. 7) there is a certaindispersion of the values obtained.

1. Method of processing polymer-based mixtures and compounds in a closedmixer (1) comprising a mixing chamber (4), a pair of rotors (2, 3), apressing ram (8) arranged above the rotors, movable between a raisedresting condition which allows the introduction of material into themixing chamber and a lowered working condition during mixing,characterized in that lowering of the ram to compress said material inthe mixing chamber occurs by controlling the position-time profile ofthe descent of the ram, until the lower end-of-stroke position isreached.
 2. Method according to claim 1, wherein control of theposition-time profile of the pressing ram (8) is performed when said ramenters into contact with the material introduced into the mixer (1). 3.Method according to claim 2, wherein control of the position-timeprofile of the pressing ram (8) is performed by regulating the controlpressure thereof, in such a way as to follow a predetermined referenceposition-time profile.
 4. Method according to claim 3, wherein the phaseof lowering the pressing ram (8) is preceded by a phase of introductioninto the mixer of at least one reinforcing filler of a polymer presentin the mixing chamber, and wherein the descent of the ram occurs duringincorporation of said filler in the polymer base.
 5. Method according toclaim 4, wherein the pressing ram reaches the lower end-of-strokeposition at the end of incorporation of the reinforcing filler in thepolymer matrix.
 6. Method according to claim 5, wherein after thepressing ram (8) has reached the lower end-of-stroke position,plasticizers of the processed polymer base are introduced into themixer.
 7. Method according to claim 6, wherein the reinforcing filler ofthe polymer base comprises at least one among silica or carbon black. 8.Method according to claim 7, wherein the position-time profile of thepressing ram (8) is a direct processing parameter.
 9. Method accordingto claim 1, wherein the material introduced into the mixer (1) comprisesa crosslinking system for a polymer-based mixture present in the mixingchamber (4).
 10. Method according to claim 1, wherein the materialintroduced into the mixer (1) comprises a mixture.